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How Venus flytraps store short-term ‘memories’ of prey

A Venus flytrap’s short-term “memory” can last about 30 seconds. If an insect taps the plant’s sensitive hairs only once, the trap remains still. But if the insect taps again within about half a minute, the carnivorous plant’s leaves snap shut, ensnaring its prey.

How Venus flytraps (Dionaea muscipula) remember that initial touch has been a mystery. A new study reveals that the plants do so using calcium, researchers report online October 5 in Nature Plants.

Scientists know that some plants have a type of long-term memory, says study coauthor Mitsuyasu Hasebe, a biologist at the National Institute for Basic Biology in Okazaki, Japan. One example is vernalization, whereby plants remember long periods of winter cold as a signal to flower in the spring. But short-term memory is more enigmatic, and “this is the first direct evidence of the involvement of calcium,” Hasebe says.

Even though the carnivorous plant, famous for its jawlike leaves, has no brain or nervous system, it can apparently count to five and distinguish between live prey and things like rain, which could inadvertently trigger its leaves to snap shut, wasting energy (SN: 1/24/16). Previous research suggested that calcium plays a role in this process, but with the help of genetic engineering, Hasebe and colleagues were able to actually see calcium in action.

After a hair inside a Venus flytrap is tapped once, calcium floods the leaves’ cells, which researchers could observe after genetically engineering plants to glow when calcium was present. A second tap, a few seconds after the first, brings more calcium into the cells, brightening the glow and causing the trap to snap shut.

The researchers added genes to the Venus flytraps that produce a protein, which glows green when exposed to calcium. When the team tapped one of the trap’s sensory hairs, the base of that hair began glowing, and then the glow spread through the leaf before beginning to fade. When the researchers touched the hair a second time — or touched a different hair on the leaf — within about 30 seconds, the trap’s leaves lit up even brighter than before, and the plant quickly snapped shut.

The results show that the flytrap’s short-term memory is a waxing and waning of calcium within leaves’ cells, the researchers say. Each time a sensory hair is triggered, it signals the release of calcium. When the calcium concentration reaches a certain level, achieved by that second, faster surge of calcium, the trap closes.

Still, the research doesn’t reveal all of the plant’s secrets. To sense prey, “the flytrap operates a fast electrical network” that can convert a fly or other insect’s movement into small voltage changes that ripple across the plant’s cells, says coauthor Rainer Hedrich, a biophysicist at the University of Würzburg in Germany. Scientists are still unsure how the calcium memory system works in tandem with that electrical network to activate the plant’s snap.

“The close association of calcium and electrical signal is known in ordinary plants, so it was also expected in the Venus flytrap,” says Andrej Pavlovič, a plant physiologist at Palacký University in Olomouc, Czech Republic who wasn’t involved in the study. But the most interesting part of the research was getting the trap to glow, he says. Such genetic transformations are common in frequently studied plants, but more difficult to do in less well-studied predatory plants, so successfully engineering the flytrap’s genes to make the plant glow “is a great leap forward in studies on carnivorous plants.”

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